A mass analyzer for the mass spectrometer typically works under a certain vacuum degree. Suitable vacuum degree ranges from 10 mtorr to 10−10 torrbased on the type of the analyzer, such as an ion trap, a quadrupole rod, a time of flight type, a Fourier transform type, etc. If the ions to be analyzed are generated in a region of relatively high gas pressure, for example an atmospheric pressure region, a series of vacuum interfaces are required to form a pressure gradient so as to effectively transfer the ions to the region of the analyzer. From the atmospheric pressure to a succeeding stage vacuum (with the gas pressure typically between 10 mtorr and 100 torr), for example, commonly-used vacuum interfaces adopt capillaries, small holes, sampling cone holes and nozzles or the combination of the above. Ion guiding devices, which may be a plurality of multipole rods applying a radio-frequency voltage or stacked-ring electrode arrays or other variants, are typically arranged behind the interfaces for transferring the ions to a succeeding vacuum interface.
In an example of the gas pressure changing form the atmospheric pressure to 1 torr, if a capillary is used as the vacuum interface, gas flow in the capillary is accelerated due to a pressure drop. After being jetted from the capillary, the gas flow forms a supersonic free expanded jet due to a sudden pressure drop. The gas is first accelerated to several times of the sonic speed rapidly and then decelerated, and forms a so-called Mach disc at a position of one time of the sonic speed. Before the Mach disc (i.e., a supersonic region), the ions are constrained within the jet, but after the ions have passed through the Mach disc, severe ion scattering occurs. Therefore, if only a radio-frequency multipole or other optical devices is used for transferring or focusing the ions after the ions have passed through the Mach disc, and it is hard to achieve a high efficiency due to a high ion-scattering speed.
Based on the conventional solution to this problem, another sampling cone hole is used for capturing part of the ions before ion-scattering occurs, methods such as the radio-frequency multipole and the like may be used for focusing the ions transferred as no dramatic and sudden gas pressure change occurs, but the sampling efficiency is very low by adopting said method. Several methods or devices have been developed in recent years. One method, which is proposed in U.S. Pat. No. 7,259,371B2 by the inventor, is that the radio-frequency multipole or other radio-frequency devices are required to constrain or focus an ion beam before the Mach disc (i.e., the supersonic free jet region), thus the ions are already in the form of a relatively focused ion beam when passing through the Mach disc, and the scattering is greatly reduced. This method may improve the ion transfer efficiency, and therefore is adopted by many commercial instruments. However, this method also has the problems that no adjustment is made to the gas flow itself on the one hand, and the focusing effect of the radio-frequency voltage is very limited under the effect of high speed gas flow, so that it is hard to ensure no ion loss. On the other hand, in this method, the most effective radio-frequency voltage is in the form of a quadrupole field in order to ensure a better compressing effect of the ion beam. However, as for the ions of a wide mass range, it is necessary to scan the voltage or frequency of the quadrupole field to obtain the maximum transmission of the ions having different mass numbers. For a non-scanning type mass analyzer such as a time of flight mass spectrum, such a method limits the efficiency of analysis.
Another device is described in patent WO2014/001827A2. The inventor believes that the ion loss is due to ion scattering caused by occurrence of turbulence at a distal end of the free jet. Therefore, a long rectifier tube may be arranged in the direction of the free jet to cause the gas flow to change from the supersonic free jet to a uniform and regular laminar flow, and then the ions are transferred along the laminar flow, thereby avoiding scattering. ADC (Direct Current) voltage or the radio-frequency voltage may be applied to the rectifier tube at the same time to obtain a better constraint on the ion beam or achieve mobility separation, etc. In order to achieve a steady subsonic laminar flow, the device needs a rectifier tube having a typical length of about 100 mm. Obviously, such a long rectifier tube is undesirable for the miniaturization of the instrument, and the ion loss caused by a long-distance transfer will increase greatly.
U.S. Pat. No. 8,269,164B2 employs another way in which a de Lavel nozzle structure is used as the vacuum interface, which may restrain free expansion of the free jet to form a collimated gas flow that may reduce the ion scattering loss. This structure is simple and small. However, according to simulations and experiments performed by the inventor, this structure tends to form a uniform and high-speed gas flow whose speed is still two times as high as the sonic speed at a distance of 100 mm from an outlet of the nozzle. In such a strong flow field, it is difficult to effectively focus the ions by application of an electric field, and the high-speed gas flow will rush into a succeeding stage vacuum, adding to the burden placed upon the vacuum pump. Naturally, an ion guide and vacuum structure with an off-axis configuration is used for separating the ions from the gas flow so as to reduce the amount of the gas flow entering the succeeding stage vacuum axially. However, introduction of the off-axis configuration significantly adds the design complexity of the interface and easily causes a phenomenon of mass discrimination due to the difference in ion mobility of different ions.